Cops are looking for a man who exposed himself to a young girl and rubbed up against her on a crowded subway in Manhattan. (400tmax/Getty Images)

A twisted sicko exposed himself to an 11-year-old girl and rubbed up against her as she was riding a downtown No. 1 train Sunday afternoon, police said.

The girl got on the subway at Penn Station with her mother and 14-year-old sister around 2 p.m.

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She told police the man sat between her and her sister, and began watching porn on a portable device.

The freaked-out girls moved to two empty seats at the end of the car, but the sicko followed, police said.

He again squeezed next to the younger girl, began touching himself, dropped his shorts and rubbed himself against her leg. The subway was crowded and their mother was distracted and didn't realize what was happening, police sources said.

The family got off the train at Christopher St. and Seventh Ave. in Greenwich Village — only to be followed by the man, cops said.

At that point, the sisters told their mom what had happened and the trio ducked into the nearest bodega to ask for help, cops said. A store clerk accompanied them back outside, prompting the pervert to bolt down Christopher St., officials said.

The man is believed to be in his 40s, stand about 5-foot-8 and weigh about 180 pounds. He was dressed in red shorts and a black tank top.